The invention relates to the hardening of edge surfaces of chain saw guide bars.
Guide bars for chain saws have long been made with hardened edge surfaces, produced by the deposition of a surface layer of a material that is harder than the guide bar body, by welding according to patents U.S. Pat. No. 2,962,812 or U.S. Pat. No. 3,241,228, or by a heat treatment. Normal carbon steel grades such as used in guide bars are conventionally hardened by being heated to a temperature above 750.degree. C., preferably around 800.degree. C., followed by rapid quenching to below 400.degree. C. The heating is conveniently done by electric induction, and the quenching by spraying with a coolant medium such as oil emulsion or cold air. The shape of the induction coil will influence the temperature distribution over the cross section, and the shape and location of coolant nozzles will influence the quenching speed, making it possible to choose the hardness distribution in the guide bar. Since the hardened edges are considerably harder than the part of the saw chain link which slides along the guide bar edges, the rate of wear of the hardened edges is reduced and occurs mainly on the links of the chain, which is of little consequence, because the links normally have a shorter lifetime for other reasons.
The higher hardness is, however, accompanied by higher brittleness and higher risk of cracking due to vibrations and bending. These cracks do not normally lead to fracture of the entire guide bar, and usually penetrate only the hard surface layer, but will then cause unevenness and rapid wear of the chain. After long use, the surface cracks can constitute starting points of deeper fatigue cracks. The largest risk of cracking occurs close to the front border of the rear part of the guide bar which is clamped between a clamping plate and the machinery part of the chain saw, since the bending moment acting on the guide bar is greatest in a narrow region R in front of and behind this border (see the conventional guide bar 10A depicted in FIG. 3), and since the guide bar in this critical region has oil supply holes and other cutouts causing stress concentration. It is thus previously taught to avoid any form of hardening in this region. The risk of cracking varies over the bar thickness, i.e., the bar dimension lying in a plane extending normal to a longitudinal axis L of the bars, and is largest at the outer corners where the edge surface meets the flat side surfaces of the guide bar.